The disclosed invention generally relates to the precision trimming of passive components in hybrid multilayer circuit structures, and more particularly is directed to a technique for trimming resistors, capacitors and other passive components buried in hybrid multilayer circuit structures.
Hybrid multilayer circuit structures, also known as hybrid microcircuits, implement the interconnection and packaging of discrete circuit devices, and generally include a plurality of dielectric layers respectively having predetermined conductor traces and conductive vias. Generally, the discrete circuit devices are mounted on the top dielectric layer.
Known techniques for fabricating multilayer circuit structures include thick film technology and dielectric tape technology. With thick film technology, each dielectric layer is individually deposited in paste form and then fired. With dielectric tape technology, each dielectric layer is made of dielectric tape. The layers may be sequentially applied and fired (referred to as the tape transfer process), or the entire laminate of layers may be fired at one time (referred to as the cofired process).
The use of precision resistors and/or capacitors with hybrid multilayer circuit structures has generally been a compromise between area available for circuit elements and discrete circuit device packing density. For example, precision resistors have been utilized in the form of either trimmable thick film resistors printed on the top dielectric layer (surface resistors) or discrete chip resistors which are mounted on the top dielectric layer along with the discrete circuit devices. Similarly, trimmable capacitors may be printed on the top dielectric layer (surface capacitors). Such use of the top dielectric layer for resistors or capacitors reduces the area that would otherwise be available for discrete circuit devices.
In search of higher packing densities, efforts have been made to include passive circuit components, such as resistors and capacitors, in the processing of multilayer circuit structures. Such components may be buried in the hybrid multilayer circuits, or they may be formed on the top dielectric layer.
However, the values of buried resistors and capacitors made pursuant to known techniques are difficult to precisely control, and therefore are unsuitable where precision resistors or capacitors are required. Moreover, buried resistors and capacitors have not been trimmable since they are buried. Surface resistors and capacitors formed on the top dielectric layer are amenable to laser trimming and are utilized as precision resistors and capacitors. Such surface resistors and capacitors are covered with a glass passivation layer, and trimming is achieved with laser equipment which cuts through the glass layer to selectively remove resistive material or capacitor plate material.
An important consideration with utilizing surface resistors and capacitors is the reduced device packing density that results from allocating area on the top dielectric layer to such passive circuit components. A further consideration with utilizing surface resistors and capacitors is the necessity of extra processing steps to provide a glass passivation layer.